1. Field of the Invention
The present invention relates to a drive circuit for a light emitting device.
2. Related Background Art
A conventional drive circuit for driving a laser diode as a light emitting device was constructed in the structure as shown in FIG. 4. Specifically, the drain terminal of PMOS-FET 41 is connected to the anode side of laser diode 40 with the cathode grounded, to drive the laser diode 40 directly. In the case of CD-R/W, DVD, etc., however, the laser diode 40 and driving IC (PMOS-FET 41 in this case) are spaced several cm or more apart from each other. In this configuration, a line 42 connects the laser diode 40 to the PMOS-FET 41. Since this line 42 definitely produces an inductance component, peaking and ringing occur because of resonance, which was a serious problem in use of products.
FIG. 5 is a drawing to show the simulation result of the conventional drive circuit for driving the light emitting device. FIG. 5 shows occurrence of heavy peaking and ringing due to resonance as described above. Efforts have been made heretofore to use wiring materials resisting the resonance, and research has been conducted on a method of interposing a resistor R and a capacitor C in series between bonding pad 43 and the ground shown in FIG. 4.
However, extra cost for the wiring materials makes it difficult to decrease the product cost. The method of interposing the resistor R and capacitor C was not a desirable method in view of yield and dispersion, either.
FIG. 6 is a diagram showing an example of a drive circuit for a laser diode by means of a PMOS-FET, and FIG. 7 a diagram showing an equivalent circuit of the drive circuit shown in FIG. 6. The result of theoretical computation of resonance constant Q in this circuit will be presented below. In the discussion hereinafter, gm1 and gm2 represent mutual conductances, gd1 a drain conductance, L an inductance, and C capacitances.                                                         g              m1                        ⁢                          V              gs1                                +                                    V              1                        ⁡                          (                                                sC                  1                                +                                  gd                  1                                            )                                +                                    (                                                V                  1                                -                                  V                  out                                            )                        sL                          =        0                            (        1        )                                                                    (                                                V                  out                                -                                  V                  1                                            )                        sL                    +                                    V              out                        ⁡                          (                                                gm                  2                                +                                  sC                  out                                            )                                      =        0                            (        2        )            
From Eq (2), we can derive V1 as follows.                                           (                                          V                out                            -                              V                1                                      )                    sL                =                  -                                    V              out                        ⁡                          (                                                g                  m2                                +                                  sC                  out                                            )                                                          (        3        )            
Vout=xe2x88x92Vout sL(gm2+sCout)+V1xe2x80x83xe2x80x83(4) 
V1=Vout {1+sL(gm2+sCout)}xe2x80x83xe2x80x83(2)xe2x80x2
By substituting (2)xe2x80x2 into (1), we can modify Eq (1) as follows.                                                         g              m1                        ⁢                          V              in                                +                                    V              1                        ⁡                          (                                                sC                  1                                +                                  gd                  1                                +                                  1                  sL                                            )                                -                                    V              out                        sL                          =        0                            (        5        )                                                                    g              m1                        ⁢                          V              in                                +                                    V              out                        ⁢                          {                              1                +                                  sL                  ⁡                                      (                                                                  g                        m2                                            +                                              sC                        out                                                              )                                                              }                        ⁢                          (                                                sC                  1                                +                                  gd                  1                                +                                  1                  sL                                            )                                -                                    V              out                        sL                          =        0                            (        6        )                                                      V            out                    ⁡                      [                                                            {                                      1                    +                                          sL                      ⁡                                              (                                                                              g                            m2                                                    +                                                      sC                            out                                                                          )                                                                              }                                ⁢                                  (                                                            sC                      1                                        +                                          gd                      1                                        +                                          1                      sL                                                        )                                            -                              1                sL                                      ]                          =                              -                          g              m1                                ⁢                      V            in                                              (        7        )            
Then we obtain Vout/Vin as follows.                                                                                           V                  out                                                  V                  in                                            =                                                -                                      g                    m1                                                                                        {                                          1                      +                                              sL                        ⁡                                                  (                                                                                    g                              m2                                                        +                                                          sC                              out                                                                                )                                                                                      }                                    ⁢                                      (                                                                  sC                        1                                            +                                              gd                        1                                            +                                              1                        sL                                                              )                                                                                                                          =                                                -                                      g                    m1                                                                                        sC                    1                                    +                                      gd                    1                                    +                                                            (                                                                        g                          m2                                                +                                                  sC                          out                                                                    )                                        ⁢                                          (                                                                                                    s                            2                                                    ⁢                                                      LC                            1                                                                          +                                                  sLgd                          1                                                +                        1                                            )                                                                                                                              (        8        )            
Assuming gm2 (=200 mS) greater than  greater than sCout (=5 mS), we obtain the following.                                                                                           V                  out                                                  V                  in                                            =                                                -                                      g                    m1                                                                                        sC                    1                                    +                                      gd                    1                                    +                                                            s                      2                                        ⁢                                          LC                      1                                        ⁢                                          g                      m2                                                        +                                                            sLgd                      1                                        ⁢                                          g                      m2                                                        +                                      g                    m2                                                                                                                          =                                                                                          -                                              g                        m1                                                              /                                          LC                      1                                                        ⁢                                      g                    m2                                                                                        s                    2                                    +                                      s                    ⁡                                          (                                                                        1                                                      Lg                            m2                                                                          +                                                                              gd                            1                                                                                C                            1                                                                                              )                                                        +                                                            gd                      1                                                                                      LC                        1                                            ⁢                                              g                        m2                                                                              +                                      1                                          LC                      1                                                                                                                              (        9        )            
From this, s, xcfx890, and Q are derived as follows.                     s        =                                                                                                  -                                          (                                                                        1                                                      Lg                            m2                                                                          +                                                                              gd                            1                                                                                C                            1                                                                                              )                                                        ±                                                                                                                                                                                                                                                                                            1                                                                                                                                                                                      Lg                                  m2                                                                                                                                              +                                                                                                                                                      gd                                  1                                                                                                                                                                                                                      C                                  1                                                                                                                                                                    )                                            2                                        -                                          4                      ⁢                                              (                                                                                                            gd                              1                                                                                                                      LC                                1                                                            ⁢                                                              g                                m2                                                                                                              +                                                      1                                                          LC                              1                                                                                                      )                                                                                                                          2                                    (        10        )                                                                                    w                0                            =                                                                                          gd                      1                                                                                      LC                        1                                            ⁢                                              g                        m2                                                                              +                                      1                                          LC                      1                                                                                                                                              =                                                                                                                  gd                        1                                            +                                              g                        m2                                                                                                            LC                        1                                            ⁢                                              g                        m2                                                                                            ≈                                  1                                                            LC                      1                                                                                                                              (        11        )                                Q        =                                            w              0                                                      1                                  Lg                  m2                                            +                                                gd                  1                                                  C                  1                                                              ≥          10                                    (        12        )            
When specific parameters are substituted into the result of inequality (12), the Q factor becomes approximately 10. With the Q factor larger than 1 as in this case, there will occur peaking and ringing as shown in FIG. 5. It is seen from the above result that it is important to set the resonance constant Q at a possible minimum value in order to restrain the peaking and ringing. For suppressing the influence of the inductance L as much as possible to control the value of resonance constant Q to near 1, it is common practice to interpose a resistor in series with L. For example, since a source follower circuit permits an is equivalent resistance to be freely controlled by electric current values, the source follower circuit is also interposed instead of the resistor in certain cases. Let us investigate a configuration incorporating the source follower circuit instead of the resistor.
FIG. 8 is a diagram showing an example of the drive circuit for the laser diode by means of a simple source follower circuit, and FIG. 9 a diagram showing an equivalent circuit of FIG. 8. The result of theoretical computation of resonance constant Q in this circuit will be provided below. In the discussion hereinafter, gm1 and gm2 represent the mutual conductances, gd1 the drain conductance, L the inductance, and C the capacitances.                                                         -                              V                gs1                                      ⁢                          g              m1                                +                                    V              1                        ⁡                          (                                                gd                  1                                +                                  sC                  1                                            )                                +                                    (                                                V                  1                                -                                  V                  out                                            )                        sL                          =        0                            (        13        )                                                                    (                                                V                  out                                -                                  V                  1                                            )                        sL                    +                                    V              out                        ⁡                          (                                                g                  m2                                +                                  sC                  out                                            )                                      =        0                            (        14        )            
Using the relation of Vgs1=Vinxe2x88x92Vout, Eq (13) can be rewritten as follows.                                                                         -                                  (                                                            V                                              i                        ⁢                        n                                                              -                                          V                      out                                                        )                                            ⁢                              g                m1                                      +                                          V                1                            ⁡                              (                                                      gd                    1                                    ⁢                                      xe2x80x83                                    +                                      xe2x80x83                                    ⁢                                      sC                    1                                                  )                                      +                                          (                                                      V                    1                                    ⁢                                      xe2x80x83                                    -                                      xe2x80x83                                    ⁢                                      V                    out                                                  )                            sL                                =          0                ,                                      (          13          )                xe2x80x2            
For Eq (14), since sCOUt≈30 mS at gm2≈200 mS and f=1 GHz, we can assume gm2 greater than  greater than sCout.                                                         (                                                V                  out                                ⁢                                  xe2x80x83                                -                                  xe2x80x83                                ⁢                                  V                  1                                            )                        sL                    ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                                    V              out                        ⁢                          xe2x80x83                        ⁢                          g              m2                                      ⁢                  xe2x80x83                =                  xe2x80x83                ⁢        0                            (        15        )                                                      (                                          1                sL                            ⁢                              xe2x80x83                            +                              xe2x80x83                            ⁢                              g                m2                                      )                    ⁢                      xe2x80x83                    ⁢                      V            out                          ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                              V            1                    sL                                    (        16        )                                                                    V              1                        ⁢                          xe2x80x83                        =                          xe2x80x83                        ⁢                                                            sL                  ⁡                                      (                                                                  1                        sL                                            ⁢                                              xe2x80x83                                            +                                              xe2x80x83                                            ⁢                                              g                        m2                                                              )                                                  ⁢                                  xe2x80x83                                ⁢                                  V                  out                                            ⁢                              xe2x80x83                            =                              xe2x80x83                            ⁢                                                (                                      1                    ⁢                                          xe2x80x83                                        +                                          xe2x80x83                                        ⁢                                          sLg                      m2                                                        )                                ⁢                                  xe2x80x83                                ⁢                                  V                  out                                                              ,                ⁢                  xe2x80x83                                              (          14          )                xe2x80x2            
By substituting Eq (14)xe2x80x2 into Eq (13)xe2x80x2, we obtain the following relation.                                                         -                              (                                                      V                    in                                    -                                      V                    out                                                  )                                      ⁢                          g              m1                                +                                                    V                out                            ⁡                              (                                  1                  +                                      sLg                    m2                                                  )                                      ⁢                          (                                                gd                  1                                +                                  sC                  1                                            )                                +                                                                      (                                      1                    +                                          sLg                      m2                                                        )                                ⁢                                  V                  out                                            -                              V                out                                      sL                          =        0                            (        17        )                                                                    -                              V                in                                      ⁢                          g              m1                                +                                    g              m1                        ⁢                          V              out                                +                                                    V                out                            ⁡                              (                                  1                  +                                      sLg                    m2                                                  )                                      ⁢                          (                                                gd                  1                                +                                  sC                  1                                            )                                +                                    g              m2                        ⁢                          V              out                                      =        0                            (        18        )            
Accordingly, Vout/Vin can be derived as follows.                                                                                                                                     V                      out                                                              V                      in                                                        =                                                            g                      m1                                                                                      g                        m1                                            +                                                                        (                                                      1                            +                                                          sLg                              m2                                                                                )                                                ⁢                                                  (                                                                                    gd                              1                                                        +                                                          sC                              1                                                                                )                                                                    +                                              g                        m2                                                                                                                                                                  =                                                            g                      m1                                                                                                                s                          2                                                ⁢                                                  Lg                          m2                                                ⁢                                                  C                          1                                                                    +                                              s                        ⁡                                                  (                                                                                                                    Lg                                m2                                                            ⁢                                                              gd                                1                                                                                      +                                                          C                              1                                                                                )                                                                    +                                              (                                                                              gd                            1                                                    +                                                      g                            m1                                                    +                                                      g                            m2                                                                          )                                                                                                                          ⁢                      
                    ≈                                                                      g                  m1                                /                                  Lg                  m2                                            ⁢                              C                1                                                                    s                2                            +                              s                ⁡                                  (                                                                                    gd                        1                                                                    C                        1                                                              +                                          1                                              Lg                        m2                                                                              )                                            +                              1                                  LC                  1                                            +                                                g                  m1                                                                      Lg                    m2                                    ⁢                                      C                    1                                                                                      ⁢                  xe2x80x83                                    (        19        )            
From this, s, xcfx890, and Q are obtained as follows.                     s        =                                                                                                  -                                          (                                                                                                    gd                            1                                                                                C                            1                                                                          +                                                  1                                                      Lg                            m2                                                                                              )                                                        ±                                                                                                                                                                        (                                                                                                            gd                              1                                                                                      C                              1                                                                                +                                                      1                                                          Lg                              m2                                                                                                      )                                            2                                        -                                          4                      ⁢                                              (                                                                              1                                                          LC                              1                                                                                +                                                                                    g                              m1                                                                                                                      Lg                                m2                                                            ⁢                                                              C                                1                                                                                                              +                                                )                                                                                                                          2                                    (        20        )                                          w          0                =                                            1                              LC                1                                      +                                          g                m1                                                              Lg                  m2                                ⁢                                  C                  1                                                                                        (        21        )                                Q        =                              w            0                                                              gd                1                                            C                1                                      +                          1                              Lg                m2                                                                        (        22        )            
From the above computation result, the resonance frequency xcfx890 increased a little, but Q itself was not affected at all. Namely, it was found that the resonance constant Q itself did not vary depending upon whether the current source was the common source of PMOS-FET or the common drain circuit of NMOS-FET, and that there was little effect thereby. Since the value of Q itself was unable to be suppressed even by the attempt to control the influence of L by the method of simply interposing the resistor R as described above, it was difficult to restrain the ringing and peaking. Since the number of portions requiring supply of electric current increased in order to solve these issues, it was also difficult to drive the circuit by the low supply voltage of 3.3 V or the like.
U.S. Pat. No. 5,898,334 discloses a method of lowering the parasitic capacitance by means of a single drive source, but this method involves such requirements that the size of MQ1 has to be small and the gate voltage has to be large. For this reason, it is necessary to use the voltage of 5 V or more, which makes driving at a low supply voltage difficult and poses the problem of heat generation.
The present invention has been accomplished under such circumstances and an object of the invention is to provide a drive circuit for a light emitting device that permits the driving at a low supply voltage, without occurrence of the ringing and peaking and with little influence of yield and dispersion.
A drive circuit according to the present invention is a drive circuit for driving a light emitting device, which comprises a first source follower circuit comprising an NMOS-FET having a gate terminal and adapted to supply a drive current to the light emitting device according to an input voltage into the said gate terminal; a second source follower circuit comprising a first PMOS-FET having a gate terminal connected to a node downstream of the first source follower circuit; and a second PMOS-FET having a gate terminal and adapted to supply an electric current to the second source follower circuit according to an input voltage into the said gate terminal, wherein a potential between the first PMOS-FET and the second PMOS-FET is supplied as the input voltage to the gate terminal of the NMOS-FET.
In this case, a voltage Vgs between the gate terminal of the NMOS-FET of the first source follower circuit and the source terminal located downstream thereof is proportional to a voltage Vgs between the gate terminal and source terminal of the first PMOS-FET having the gate terminal connected to the node downstream of the first source follower circuit. Therefore, while an electric current flowing in each MOS-FET is determined according to the voltage Vgs between the gate terminal and source terminal, the electric current flowing in the first PMOS-FET is in a proportional relation to the electric current flowing in the NMOS-FET. On the other hand, the electric current flowing in the first PMOS-FET is determined according to the input voltage into the gate terminal of the second PMOS-FET. Accordingly, the electric currents flowing in the first PMOS-FET and the NMOS-FET become constant if a fixed voltage is applied to the gate terminal of the second PMOS-FET.
The potential at the node between the downstream side of the NMOS-FET of the first source follower circuit and the light emitting device can vary depending upon states of the light emitting device and peripheral circuits, but even with such variation the electric current flowing in the NMOS-FET rarely varies as long as the electric current flowing in the first PMOS-FET is kept constant. Since the Q factor of the circuit varies depending upon the electric current flowing in the NMOS-FET, if the constants of the circuit components are selected so as to make the Q factor low, use of the drive circuit of the present configuration makes it feasible to maintain the Q factor at a low level.
Since the resonance constant Q can be made small in the present drive circuit as described above, it becomes feasible to suppress the ringing and peaking and drive the light emitting device on a stable basis. Since the number of components can be reduced, it is feasible to reduce the influence of yield and dispersion and reduce the cost. Since the impedance is low, the gate voltage can also be set low, thus enabling the driving at the low supply voltage. Further, since there arises no problem even with some parasitic capacitance, there is no need for employing the configuration for lowering the parasitic capacitance as before.
In the drive circuit for the light emitting device, it is preferable to gradually apply the electric current to the light emitting device, for example, in a stepped pattern of about four steps, instead of increasing the electric current directly to a high level.
Then the drive circuit for the light emitting device is characterized by comprising a PMOS-FET group for further supplying a drive current to the light emitting device through the node downstream of the first source follower circuit. Namely, when the drive current is supplied from the PMOS-FET group to the light emitting device, the total of the drive current supplied to the light emitting device can be increased.
When the drive current is supplied to the light emitting device, for example, by four steps in this configuration, the drive current of the first step is given by use of the first and second source follower circuits and the second PMOS-FET, whereby it becomes feasible to make the resonance constant Q small, suppress the ringing and peaking, and implement the stable driving of the light emitting device. In this case, drive current increases of the rest three steps can be implemented by sequentially activating the PMOS-FET group. Since the present configuration obviates the need for using the aforementioned Q-factor variation limiting structure in the PMOS-FET group, it is feasible to decrease the number of components, reduce the influence of yield and product dispersion, and decrease the cost.
Since the impedance is low, the gate voltage can be set low, thus enabling the driving at the low supply voltage. Further, since there arises no problem even with some parasitic capacitance, there is no need for employing the configuration of lowering the parasitic capacitance as before.
In the present drive circuit for the light emitting, the mutual conductance of the first source follower circuit comprising the NMOS-FET has either value in a range of 10 mS (millisiemens) to 100 mS.
Since the mutual conductance of the source follower circuit comprising the NMOS-FET has either value in the range of 10 mS to 100 mS, the resonance constant Q can be made small.
A light emitting apparatus according to the present invention comprises a light emitting device and the drive circuit for the light emitting device in either of the configurations as described above.
This configuration makes it feasible to suppress the ringing and peaking and implement stable light emission. Since the number of components can be reduced, it becomes feasible to reduce the influence of yield and dispersion and decrease the cost. Since the impedance is low, the gate voltage can be set low, thus enabling the driving at the low supply voltage.